A) Field of the Invention
The present invention relates to a microlens array manufacture method which transfers lens shapes to a transparent substrate or underlying layer by etching using a resist pattern as a mask, and to a microlens array produced.
B) Description of the Related Art
A microlens array having regularly aligned microlenses is used in a photocoupler for an optical fiber array, an optical integrated circuit, a solid state imager, an optical system of an electronic copy machine, a liquid crystal display and the like. According to a conventional manufacture method for such a microlens array, circular positive resist patterns are formed and changed to convex lens shapes by heating and reflow process, and thereafter the convex lens shapes of the positive resist patterns are transferred to a substrate by a dry etching process (for example, refer to Japanese Patent Laid-open Publication No. HEI-7-174903).
With this method, as shown in FIG. 7, on one principal surface of a quartz substrate 1, a necessary number of circular positive resist patterns 2a to 2d are disposed in line. Each resist pattern is spaced from other resist patterns so as not to contact each other. The regions outside the circular patterns 2a to 2d are exposed to leave un-exposed circular patterns. Next, the resist patterns 2a to 2d are heated and reflowed to form convex lens shapes (spherical convex shapes) by a surface tension. Thereafter, by using the resist patterns 2a to 2d as a mask, the substrate is dry-etched to transfer the lens shapes of the resist patterns to the principal surface of the substrate. In this manner, a microlens array can be formed on the substrate 1, having convex lenses of circular plan shapes linearly aligned in correspondence with the resist patterns 2a to 2d. 
Generally, a larger exposure energy is required at a narrower exposure line (or area) width, when resist patterns are formed by subjecting a positive resist layer to exposure and development process. If the exposure area is narrower, a higher exposure energy is needed to transfer the mask image at a high fidelity. In the example shown in FIG. 7, a larger exposure energy is required at a position nearer to the center line Lc intersecting the centers of the resist patterns 2a to 2d. An exposure process is performed at such an exposure energy as proper resist patterns are formed on the center line Lc. With this setting, although proper exposure is performed on the center line Lc along an X direction, the exposure line (or area) width becomes broader at a position remoter from the center line Lc in a Y direction perpendicular to the X direction, resulting in excessive exposure. Proper resist patterns in conformity with the reticle patterns cannot be obtained.
FIG. 8 shows cross-section of a convex lens La formed on the substrate 1 by transferring the resist pattern 2a, the convex lens La corresponding to the resist pattern 2a formed by the above-described method. Lx and Ly indicate the cross section of the lens La along the X and Y directions. As described above, although the lens pattern 2a is subjected to proper exposure on the center line Lc along the X direction, excessive exposure is performed along the Y direction away from the center line Lc. The lens La is not a perfect circle, and the radius of curvature in the X direction becomes larger than that in the Y direction. Thus, a focal point Py in the Y direction becomes nearer to the substrate than a focal point Px in the X direction. A focal length difference (astigmatism) between the sagittal direction (X direction) and the meridional direction (Y direction) becomes large so that the optical characteristics are degraded.